$\dfrac{7}{10} - \dfrac{5}{8} = {?}$
$ = {\dfrac{7 \times 4}{10 \times 4}} - {\dfrac{5 \times 5}{8 \times 5}} $ $ = {\dfrac{28}{40}} - {\dfrac{25}{40}} $ $ = \dfrac{{28} - {25}}{40} $ $ = \dfrac{3}{40}$